There are several well known methods for implementing the transmit/receive coupler in a transceiver which both transmits and receives. For example, a Single Pole Double Throw (SPDT) switch is employed in a wide range of applications where talk and listen do not occur simultaneously, as with most transceivers using Time Division Multiplexing (TDM). Simultaneous transmit and receive (STAR) has been accomplished using a frequency diplexer, circulator or hybrid coupler as illustrated in FIG. 1. This can be combined with Code Division Multiple Access (CDMA).
A frequency diplexer can often handle high power and works well for frequency separations of 10 MHz or more. However, it is not suitable for waveforms with overlapping transmit and receive spectra. STAR implementations using a circulator or hybrid coupler generally require excellent antenna matching to minimize leakage of transmit signal into the receiver caused by antenna reflection. This reflection is difficult to achieve in practice and becomes more difficult as operating bandwidth increases.
FIG. 1 illustrates a block diagram of a radio 100 that is receiving signal 118 from transmitter 119 and simultaneously interacting with a hostile radio 117 through transmit signal 116. Signal 116 could be intended to disrupt communication between 119 and 117 in some situations. The transmitter circuit includes broadband signal generator 110 and power amplifier 111 which produces transmit signal 113 with a power of 10-100 W or more. Antenna coupler 103 directs the output from power amplifier 111 (via port 103b) to the antenna match 102 and antenna 101 (via port 103c) where it is radiated as signal 116. In some cases, antenna match 102 may be integral to antenna 101. Antenna 101 also receives a desired signal 117 from transmitter 119. The signal received by antenna 101 from transmitter 119 is conveyed through antenna coupler 103 to the input of receiver subsystem 112 (via port 103a).
The input signal 114 consists of reflection of broadband transmit waveform 113 overlapping with a narrow band or broadband desired received waveform 118. The receiver input signal 114 appearing at coupler port 103a consists of the desired signal 118 as well as a reflected signal created from signal 113 by antenna and coupler mismatch. FIG. 2 illustrates the spectral components of the two signals comprising receiver input signal 114. Item 200a in FIG. 2 represents a small narrowband received signal 202 derived from signal 118 and a larger broadband signal 201 created by reflection of signal 113 by imperfect antenna matching. Item 200b in FIG. 2 illustrates the corresponding spectrum for a small broadband received signal 203 derived from signal 118.
FIG. 2A is a graph of the measured reflection from a small antenna suitable for 0.8-1.8 GHz operation. As mentioned above, minimizing leakage of transmit signal into the receiver caused by antenna reflection is difficult to achieve in practice, with typical performance illustrated in FIG. 2A. This represents power in signal 113 that is reflected by the antenna and would pass from the antenna port of coupler 103 to the receiver as an interfering signal. The reflected power varies from 0.3% to 10% of the power in transmit signal 113. The total reflected power for a transmit signal 113 with uniform power over 0.8-1.8 GHz is 4%. The total reflected power increases to 7% for a waveform spanning 0.7-2.4 GHz. Reflection of this magnitude will generally overload or damage most receiver subsystems when the power of signal 113 is 10-100 W.
Various implementations of FIG. 1 for radar and Joint Counter Radio controlled Improvised Explosive Device Electronic Warfare (JCREW) applications are being developed. Recent work in this field has focused on new ways to implement coupler 103 with wider bandwidth and reduced internal transmit-receive leakage. This work assumes the antenna is well matched and therefore has not addressed the problem of interference from antenna reflection.
Hence, the existing solutions do not address the problem of isolating a high power signal from a low power signal present on a single antenna when the two waveforms have overlapping spectra and the antenna match is imperfect. The high power leakage signal substantially degrades the signal-to-noise ratio (SNR) of the low power received signal due to transmit signal phase noise and receiver dynamic range limitations. Digital Signal Processing (DSP) methods are useless if the receiver Low Noise Amplifier (LNA) or mixer is over-driven, because the subsequent analog to digital conversion (ADC) operations will either be capturing a distorted or saturated signal in which the information required for DSP operation is unrecoverable or missing, or the LNA or mixer will be damaged altogether.